In the past, slurry trench techniques, as disclosed in Brunner British Pat. Nos. 913,527 and 913,528 and, Veder U.S. Pat. No. 3,310,952, have been used in the construction of tunnels and, in one example, a pair of slurry trenches forming the sidewalls are formed in panel sections and in which the concrete forming the sidewalls is cast in situ forming two spaced apart sidewalls for the tunnel, the concrete being preferably steel reinforced by steel cages and the like. The tunnel roof may then be cast directly on the unexcavated tunnel and spanning the space between the previously cast concrete walls. Then the tunnel itself is excavated under the cast concrete and the floor or invert is poured. Alternatively, the walls may be precast panels and lowered into the slurry trench excavation with grouting inserted between the earth walls. The wall panels may extend between solidier beams or concrete columns as shown in Miotti U.S. Pat. No. 3,139,729, and interlocked to form the tunnel walls. Thereafter, the roof and floor may be cast as previously described.
According to the present invention, tunnels, particularly shallow tunnels, are constructed by excavating slurry filled cross-trenches in a direction transverse to the axis of the tunnel and at regular intervals along the line or axis of the tunnel. Prefabricated frame elements are inserted into the cross-trenches to, in essence, create the tunnel wall lateral support structure before the tunnel walls are installed. Then the tunnel sidewalls are excavated under bentonite clay or mud slurry to form cross-slots between the previously installed support frames. Then the precast concrete sidewalls or panels are inserted between the support frames. The frames and sidewalls have interlocking tongue end groove or keyway structures so as to interfit and lock same into position. When the sidewalls are in place the roof of the tunnel may be either cast in place and then the soil excavated to form the actual tunnel and then the floor cast or the tunnel may be excavated down to the floor or invert level level and then the floor or inverts cast and then the roof cast.
By following the invention, the tunnel can be constructed faster and less expensively and result in a significantly improved tunnel product. Since the panel sides and the support frames are precast or prefabricated, it makes for greater uniformity, reduction in cost, better surfaces and also reduced sections (resulting in a saving of materials) this is because many times what happens in the slurry wall excavation, and practically speaking, there are not excavating tools less than 2' wide which are commercially available. The precast steel reinforced concrete side panel walls or planks constituting the sidewalls of the tunnel can be 10" or 12" thick and with the excavation tools being approximately 2' wide, the oversize permits the aligning of the panels and the frames perfectly because it allows some play. Moreover, the big oversize allows the grouting to be placed in the space between the outer sidewall of the panels and the frames and the remaining earth wall forming the tunnel resulting in an architecturally much better job. Moreover, it is advantageous to prefabricate and precast the support frames and wall panels since this enables a much better quality control of the structure that is going to be put into the tunnel and they can be positioned perfectly so that the natural roughness of the slurry wall cast in situ is eliminated. The basic concept therefor is the concept of putting in prefabricated concrete to support frames in a slurry trench transversely to the excavation or tunnel direction and in essence creating a support structure before the tunnel side (as well as roof and floors) walls are installed.